It doesn’t help. Without knowing what the API expect (array, string, object…) I cannot tell you exactly what you need to do.
Could be easy as adding [ square bracket] around (if the api expect an array of string) or more complex to use :format as text on the list or this could be :join with , and :format as json-safe if this expect a string.

So my question is what did you provide in API Connector for this field when you initialized it? Or API Doc that can show what API expect there…